Field of the Technology
The present disclosure relates to mechanical fasteners and fastener stock, and in particular to fasteners and fastener stock comprising alpha/beta titanium alloys.
Description of the Background of the Technology
Titanium alloys typically exhibit a high strength-to-weight ratio, are corrosion resistant, and are resistant to creep at moderately high temperatures. For these reasons, titanium alloys are used in aerospace and aeronautic applications including, for example, landing gear members, engine frames, and mechanical fasteners.
Reducing the weight of an aircraft results in fuel savings, and thus there is a strong drive in the aerospace industry to reduce aircraft weight. Titanium and titanium alloys are attractive materials for achieving weight reduction in aircraft applications because of their high strength-to-weight ratio. Currently, titanium alloy fasteners are used in less demanding aerospace applications. In certain aerospace applications in which titanium alloys do not exhibit sufficient strength to meet the particular mechanical requirements of the application, heavier iron and nickel based alloy fasteners are used.
Most titanium alloy parts used in aerospace applications are made from Ti-6Al-4V alloy (ASTM Grade 5; UNS R56400; AMS 4965), which is an alpha/beta titanium alloy. Typical minimum specification for small diameter Ti-6Al-4V fastener stock, i.e., fastener stock having a diameter less than 0.5 inches (1.27 cm), are 170 ksi (1,172 MPa) ultimate tensile strength (UTS), as determined according to ASTM E8/E8M-09 (“Standard Test Methods for Tension Testing of Metallic Materials” ASTM International, 2009), and 103 ksi (710 MPa) double shear strength (DSS), as determined according to NASM 1312-13 (“Method 13-Double Shear”, Aerospace Industries Association—National Aerospace Standard (Metric), Feb. 1, 2003).
Iron and nickel based superalloys, such as, for example, A286 iron-base superalloy (UNS S66286), are representative of materials used in aerospace fastener applications having the next tier of strength. Typical specified minimum strengths for cold drawn and aged A286 alloy fasteners are 180 ksi (1,241 MPa) UTS and 108 ksi (744 MPa) DSS.
Alloy 718 nickel based superalloy (N07718) is a material used in aerospace fasteners that represents the uppermost tier of strength. Typical specification minimums for cold drawn and aged Alloy 718 superalloy fasteners are 220 ksi (1,517 MPa) UTS and 120 ksi (827 MPa) DSS.
In addition, two beta titanium alloys that currently are in use or are under consideration for use as high strength fastener materials exhibit minimum ultimate tensile strength of 180 ksi (1,241.1 MPa) and minimum DSS of 108 ksi (744.6 MPa). SPS Technologies, Jenkintown, Pa., offers a titanium alloy fastener fabricated from an optimized beta-titanium alloy that conforms to the chemistry of Ti-3Al-8V-6Cr-4Zr-4Mo titanium alloy (AMS 4958). The SPS bolts are available in diameters up to 1 inch (2.54 cm). Alcoa Fastening Systems (AFS) has developed a high-strength titanium fastener made from a titanium alloy that conforms to the nominal chemistry of Ti-5Al-5Mo-5V-3Cr-0.5Fe titanium alloy (also referred to as Ti-5553; UNS unassigned), a near beta-titanium alloy. The AFS Ti-5553 alloy fasteners reportedly exhibit tensile strength of 190 ksi (1,309 MPa), greater than 10% elongation, and minimum DSS of 113 ksi (779 MPa) for uncoated parts and 108 ksi (744 MPa) for coated parts.
Beta-titanium alloys generally include a high alloying content, which increases the cost of components and processing compared with alpha/beta titanium alloys. Beta-titanium alloys also generally have a higher density than alpha/beta titanium alloys. For example ATI 425® alpha/beta titanium alloy has a density of about 0.161 lbs/in3 (4.5 g/cm3), whereas the beta-titanium alloy Ti-3Al-8V-6Cr-4Zr-4Mo has a density of about 0.174 lbs/in3 (4.8 g/cm3), and the near beta-titanium alloy Ti-5Al-5Mo-5V-3Cr-0.5Fe has a density of about 0.168 lbs/in3 (4.7 g/cm3). Fasteners made from titanium alloys that are less dense may provide further weight savings for aerospace applications. In addition, the bimodal microstructure that is obtained, for example, in solution treated and aged alpha/beta titanium alloys may provide improved mechanical properties such as high cycle fatigue compared to beta-titanium alloys. Alpha/beta titanium alloys also have a higher beta transus temperature (Tβ) than beta-titanium alloys. For example, the Tβ of ATI 425® alpha/beta titanium alloy is about 1,800° F. (982.2° C.), whereas Ti-5Al-5Mo-5V-3Cr-0.5Fe beta titanium alloy has a Tβ of about 1,500° F. (815.6° C.). The difference in Tβ for the two forms of titanium alloys allows for a larger temperature window for thermomechanical processing and heat treatment in the alpha/beta phase field for alpha/beta titanium alloys.
Given the continuing need for reduced fuel consumption through aircraft weight reduction, a need exists for improved lightweight fasteners for aerospace applications. In particular, it would be advantageous to provide lightweight alpha/beta titanium alloy aerospace fasteners and fastener stock exhibiting higher strength than current generation aerospace fasteners fabricated from Ti-6Al-4V alpha/beta titanium alloy.